The present invention relates to a method for production of safety/rupture discs, namely discs or dome-shaped elements having a pre-calculated breaking threshold.
In particular, the present invention is advantageously used in producing devices applied to plants along pneumatic or hydraulic lines to ensure the necessary safety measures of the plant itself which can be submitted to conditions different from the design ones.
More specifically, such devices are applied to emergency lines to fully close the duct on which they are applied.
A high-pressure fluid acts in one of the half-spaces defined by the device, while the opposite half-space is acted upon by a lower pressure, for example the atmospheric one.
Alternatively, the safety discs of known type can protect plants operating at sub-atmospheric pressure. In this case too, the device is submitted to a pressure difference acting between the two opposite faces of same.
When the difference between the pressure acting inside the plant and the external pressure exceeds a value of pre-calculated safety threshold, this disc opens by tearing and allows the fluid under pressure to go beyond the safety device so as to flow out of the plant. In this manner, a possible and undesirable pressure excess is not able to damage other parts of the plant.
In case of plants operating at a sub-atmospheric pressure, rupture of the safety disc enables the fluid under atmospheric pressure to flow into the plant.
Typically, the safety discs to which reference is made have a disc-shaped configuration with a surface on which the fluid under pressure acts, which surface can be flat, concave or convex.
Alternatively, said discs have a square or rectangular shape depending on the duct onto which they have to be applied.
The discs that do not have any frangible line of pre-established rupture, open in a non-definite manner with possible separation of some parts. For this reason, in many cases preferably discs with frangible lines of pre-established rupture are used, which lines can be obtained with notches or cuts of various shapes, which can be through cuts or not. In the case of through cuts a second continuous layer, weaker than the calibrated section is necessary, to ensure pneumatic and/or hydraulic tightness. When there are no through cuts this is not required.
At all events, these discs can have a plurality of non-through cuts disposed on a surface of the disc in a suitable configuration so as to define corresponding frangible lines of pre-established rupture.
In other words, when overcoming of the threshold value of the fluid pressure occurs, the safety disc tears at said frangible lines of pre-established rupture.
In the methods for producing these discs, particular attention must be paid exactly to the execution of said non-through cuts in terms of depth, length and width, but also as regards maintenance of the mechanical features of the disc material close to the non-through cuts. In particular, it is necessary for this material to keep its crystalline-metallurgic structure unchanged.
In fact, the non-through cut sizes are selected as a function of the value of the pre-established pressure threshold to which the disc must break. Undesirable mechanical alterations and/or alterations in the crystalline structure that on the other hand can be hardly evaluated, can modify this threshold value. In this manner, the safety disc starts operating before or after achievement of this value.
In known production methods, cuts are mainly made mechanically. In particular, cuts can be made by permanent set through use of punches and related counter-punches or moulds and counter-moulds.
Alternatively, cuts are made by micro-milling. In other words, a portion of the material is mechanically removed with great accuracy.
The described methods at all events involve use of tools coming into contact with the mentioned safety disc.
First of all, this type of working imposes reduced work speeds. In addition, similar production methods are expensive, without on the other end ensuring excellent accuracy levels.
Finally, the direct contact between the tools required for working, and the disc involves gradual decay of the tools that must be reground and/or replaced bringing about, as a result, production stops and impairing constancy in the discs produced at a later time. All that obviously affects costs and production.
To at least partly obviate the mentioned drawbacks, cutting methods are known that use a laser beam. In this type of method described in the US Patent Application No. 2006/0237457, the disc is previously covered with a layer of protective material. In this case, the laser beam traces the lines to be cut directly in the protective layer and not in the metal. The cuts are then actually made by an electrical-polishing process. Removal of the metal material takes place by an electrolytic method (electrical polishing). Finally, the protective material layer is removed.
However, this type of working involves very long work times and high production costs due to the indispensable requirement of applying and removing the protective layer.
The use of laser beam for directly machining the non-through cuts on the disc has been discussed in various publications in recent years. It has been well established that laser machining is a material removal process based on transportation of photon energy into the target material. This energy removes material by two physical effects: by melting the material or through direct vaporization or ablation.
Document US2006/0237457 discloses that using a laser beam directly on a disc for providing non-through cut is not satisfactory for a number of reasons, mainly because lasers significantly heat and burn the disc, oxidize the material and change the metallurgy of the metal. Therefore, according to this publication, it is impossible providing non-through cut on the disc through direct vaporization or ablation of the material, namely annulling or rendering irrelevant the material melting.
However, document PCT/US2009/059178 states that laboratory tests have shown that, when using laser scoring with an adequate selection of laser and process parameters, the heat-affected zone is narrow and the resolidified layer is of micron dimensions. Distortion is therefore negligible. According to this publication, various tests have shown that the heat-affected zone is of such small dimensions that it does not affect negatively the overall mechanical characteristics or performance of the disc for the range of commercial applications typical of a rupture disc device.
In particular, publication PCT/US2009/059178 teaches that it is possible to remove material from the rupture disc as vapor without melting or oxidizing disc material adjacent the removed material by selecting a wavelength of the laser beam within a range of between approximately 200 nanometers and approximately 1064 nanometers, by selecting a pulse repetition rate within a range of between approximately 2 kHz and approximately 10 kHz, by selecting a speed of relative motion between the laser and the rupture disc for each pass of the laser within a range of between approximately 10 mm/s and approximately 60 mm/s, by selecting a laser having a power within a range of between approximately 0.5 Watts and approximately 30 Watts and by selecting a focal spot diameter of the laser within a range of between approximately 20 μm and approximately 100 μm.
By using the above ranges for producing rupture discs the average score depth per pass is between 6 μm and 168 μm (depending on the actual combination of parameters).
The Applicant has noted that, in order to obtain a suitable rupture disc the total passes should be more than one, for example five. In fact, one pass only cannot guarantee the required depth uniformity throughout the entire frangible line.
It is to be noted that the above score depth per pass is unsuitable in case very thin discs have to be produced.
In fact, there is a need for producing rupture discs having thickness of about 20 μm with a total score depth of about 10 μm. Even taking into consideration the minimum score depth per pass of about 6 μm, five passes would result in 30 μm total score depth (clearly unsuitable for producing very thin discs).
Moreover, it is not always allowable (in terms of performance of the rupture disc) having resolidified layer of micron dimensions in the proximity of the non-through cut.